Flue gases contain numerous gaseous and solid contaminants. Gaseous contaminants include NOx, SOx, H2S, COS, and carbonyl. These gaseous contaminants are typically removed with a re-circulating solvent system, which can include MEA, DGA, DIPA, MDEA, propylene carbonate, tributyl phosphate, normal methyl pyrrolidone, as well as various polyethylene glycol dialkyl ethers. Mixtures of these solvents are also used, including for example, mixtures of dimethyl ethers of polyethylene glycols and water. Solvents are typically recirculated, with 10% to 20% of the circulating solvent commonly being filtered in a slip stream.
Solid contaminants include calcium and magnesium carbonates or sulfates, metal sulfide, iron carbonate, iron oxide, and mill scale. Unless removed, these and other solids tend to foul columns, vessels, heat exchangers, and carbon bed filters. They can also erode the protective iron sulfide films from internal piping surfaces, thereby undesirably accelerating pipe erosion. Although solid contaminates can generally be moved via filtering, they can be saturated with hazardous gases such as H2S, COS, and carbonyl. The maintenance of the filters thus requires special handling equipment and procedures. Improper handling of such contaminates can potentially endanger operating personnel and contaminate the environment.
Disposable media, including cartridges, pre-coat filters, and throw away bags, are often used to collect solid-contaminants. But the use of disposable media entails repeated media purchases and potentially costly installation. Moreover, replacement of filter cartridges and other disposable media often requires special procedures due to the presence of hazardous gases in the filtered materials. In addition, disposable media are themselves waste products, and pose their own waste disposal issues.
In an attempt to mitigate the problems associated with the use of disposable filter media, some petroleum refiners and chemical processors are currently switching to cleanable media filters. Although contemporary clean-in-place filters are capable of removing solids and particles from a solvent, and although the cleaning process can be automated to some extent to avoid the hazards and costs associated with the manual replacement of filter elements, the contemporary backwashing process in such systems also tends to undesirably remove valuable solvents from the system. While the loss of such solvents during the backwashing operation can be acceptable for some small plants, it becomes very costly and economically prohibitive for larger plants, especially for expensive solvents. Moreover, currently known systems often liberate considerable quantities of displacement vapors into the atmosphere where a gas backwash is employed.
Thus, there is still a need to provide improved filter systems and methods, where the undesirable loss of solvent is substantially mitigated, and wherein displacement vapors remain contained with in the system.